1. Field of the Invention
The present invention relates to the field of motor control, and more particularly, to a method of fastening in position a sensor to a motor stator of a distributed-wound brushless DC motor and a sensor fastening frame.
2. Description of Related Art
Motors have developed from the earliest DC and AC motors to modern brushless DC motors (BLDC). A general DC motor with carbon brushes uses the brushes to transmit DC power to the commutator. The commutator then guides the current to the coils on the rotor to generate electromagnetic force, which produces attractive and repulsive effects with the stator on the permanent magnet thereby inducing rotation. Change of the direction of the electromagnetic force relies on the change of carbon brushes in contact during the rotation of the commutator (the commutator being installed on the rotor and rotating with the rotor). Different carbon brushes have different polarities, allowing change in the current direction. Many disadvantages result from the use of carbon brushes utilized this way such as friction causing unnecessary mechanical energy loss due to contact with commutator. The materials in contact increase the overall resistance, which in turn decreases power transmission. Also, current alternates on the contact surfaces, thereby generating sparks which lead to electronic interference. Furthermore, cleaning and replacement of carbon brushes are labor intensive and costly.
As for the principles of an AC motor, alternating current is responsible for alternating the polarities to introduce alternating current into an outside stationary stator having coils to produce a rotating magnetic field. Such a magnetic field then generates rotations of the permanent magnet on the inside rotor. An AC motor completely overcomes the drawbacks caused by contact between carbon brushes and the commutator in a DC motor and is currently the most widely used electric motor with high output efficiency. However, it is difficult to control the speed of an AC motor. Speed changes depend on control of the alternating current frequency, while voltage changes only result in torque changes. Also, AC motors aren't directly applicable in applications needing direct current such as IT products and electric cars.
Brushless DC motors inherit the advantages of the above two types of motors. The brushless design eliminates trouble caused by frictions due to contact and the use of direct current allows easier control. The principles and structure of a brushless DC motor are very similar to the aforementioned AC motor. Basically, an rotor is made up of a permanent magnet. As long as the magnetism remains unchanged, the stator is then identical to that of the DC motor. Magnetic force is generated via coils.
FIGS. 1 and 2 are diagrams illustrating the operating principles of a simple brushless DC motor. As shown in FIG. 1, an rotor 10 is made of a permanent magnet presenting different polarities at each end. A stator 11 and a stator 12 are general electromagnetic coils, which in turn generate magnetic force via current flow. The labels N and S on the drawings represent the north and south poles of the magnetic field, respectively. The motor of FIG. 1 is in its initial state. Current is introduced from the top of the stator 11 and the stator 12. Arrows on the coils refer to the directions of current flow, wherein the generated magnetic fields mutually attract the poles of the rotor magnet having opposite polarities, thereby causing both ends of the rotor 10 to move closer to the stators 11, 12, thus effecting clockwise rotation.
As shown in FIG. 2, the rotor 10 rotates from the position shown in FIG. 1 to the position where it center is aligned with the center points of the stators 11, 12, whereupon the direction of current is then reversed to input current from the bottom. The magnetic field direction of the electromagnets consequently reverses, repulsively pushing the two ends of the rotor 10 away from the stators 11, 12, as shown in FIG. 2, continuing rotation of the rotor 10 in a clockwise direction. This method of using alternating current input directions to change the direction of the magnetic fields continues until smooth rotor 10 rotation is achieved. In practice, an actual motor has a more complex design and more stators are installed to increase its performance.
In order to generate an appropriate direction of the magnetic field in accordance with the rotor 10 position, a Hall sensor is utilized to ascertain the rotor 10 position. A Hall sensor is a sensing component that detects the direction of the magnetic field. Its working principle is well known and will not be elaborated on. The structure and method are only discussed in terms of Hall sensor installation.
FIG. 3 illustrates a structure of a stator of a conventional brushless DC motor. The stator adopts the method of concentrated coils, whereby wires 20 are wound on individual fixed cores at preset positions of the stator 21. The coils formed by winding the wires 20 on each individual fixed core do not overlap, so called concentrated coils. Insulating frames 22 protruding above and below the wires are installed on the inner and outer rings of the stator 21, and may be used to support and/or attach Hall sensors as explained subsequently. As shown in FIG. 4, the stator 21 and a Hall sensor PCB 24 are installed in the conventional brushless DC motor in accordance with the following sequence: the stator 21 with insulating frames 22 is first put into a casing 25; BLDC rotors 23 are subsequently inserted; and finally the Hall sensor PCB 24 is installed on the top end of the insulating frames 22.
However, such a Hall sensor board is only appropriate for new molded stator structures because modern brushless DC motors are still in the early development stage and there are only a few standardized components on the market. Using concentrated coils to manufacture a brushless DC motor has the advantages of a lower overall thickness and a simplified magnetic field control, but efficiently manufacturing motors does not just involve the motor design but also the manufacturing techniques. In order to manufacture high-speed motors, companies have to invest a significant amount of capital in developing machines and molds. Also, in order to achieve an effective production scale to lower the cost, companies are reluctant to change the existing standard production flow.
Hence, based on the similarities of the structures of brushless DC motors and AC motors, a technique of installing AC motor stators in brushless DC motors has developed, called distributed-wound brushless DC motors. As shown in FIG. 5, a conventional AC motor stator is illustrated. The coil winding method adopted is the distributed coil winding method. Wires 40 overlap one another on a stator 42 (only partial wires are shown for clarity), which is a totally different technique compared to the previous technique of using concentrated coils. The advantages of distributed coils are well known and will not be elaborated.
However, the most difficult part in the application of the motor stator 42 to the brushless DC motor is that the protruding height 41 of the distributed coils obstructs the installation of Hall sensors, and the motor stator 42 also lacks the corresponding structure for installing Hall sensors.
In summary, in view of the drawbacks of the conventional techniques and practical manufacturing limitations, it is a critical challenge for designers of motors to develop a structure and a method for fastening Hall sensors to the traditional AC motor stator structure without changing the existing motor manufacturing machines and established standard procedures, thereby allowing low-cost, reliable traditional AC motor stators to serve as components of brushless DC motors.